This disclosure relates to polymer ferroelectric materials, and more particularly, to processes for removing photoresist and residues during the fabrication of integrated circuits employing ferroelectric materials.
FeRAM devices are a non-volatile form of memory that utilizes a thin film of a ferroelectric material as a capacitor dielectric. The ferroelectric materials generally have a dielectric constant in the hundreds or thousands at room temperature and two stable remanent polarization states. Ferroelectric materials include ceramics, polymers, and some minerals, wherein each type of material has a crystal structure that can be electrically polarized and that polarization can be reversed when the proper electrical field is applied. Once a ferroelectric material is polarized, all the molecules are aligned so that one side of the crystal is positively charged and the other side is negatively charged. Reversing the polarization can occur by connecting the ferroelectric material to a battery or other power supply.
It is known that ferroelectric materials are generally sensitive to elevated temperature exposure as well as to chemical attack, which ferroelectric materials are frequently exposed to during fabrication of FeRAM devices, e.g., during etching, annealing, cleaning, or the like. For example, above a certain temperature, called the Curie temperature, the heat breaks forces aligning the atoms, which causes the crystal structure to become more symmetric resulting in a loss of the ferroelectric polarizability. The Curie temperature for each ferroelectric material is different and can even be lower than room temperature. In this last case, the crystals would be symmetrical at room temperature, but cooling them below their Curie temperature would change them to the ferroelectric state.
The nonvolatile memory device with the ferroelectric thin film uses the principle that if an electric field is applied to the device to adjust the orientation of polarization and to input a signal, then the orientation of remanent polarization remained when the electric field is removed makes the digital signal 1 or 0 be stored in the device. As such, the FeRAM devices can be used to store information at power off conditions and is comparable in operating speed to the conventional DRAM.
Ferroelectric random access memories that incorporate capacitor thin films having ferroelectric properties are becoming more common. The use of a ferroelectric material as the capacitor thin film in place of a conventional silicon oxide film or a silicon nitride film provides improved low-voltage and high-speed performance. Further, the residual polarization of the ferroelectric materials means that FeRAMs do not require a periodic refresh to prevent loss of information during standby intervals like conventional dynamic random access memory (DRAM). FeRAMs also provide this non-volatile performance without requiring the more complex structure of a conventional SRAM, thereby allowing increased densities.
In a conventional fabrication method of a FeRAM device, a bottom electrode, a ferroelectric thin film, and a top electrode are formed on the semiconductor substrate. Each electrode is lithographically patterned using a photoresist masking layer and a dry etching process to form individual memory pixels. The dry etching process is typically performed by a plasma mediated process such as by RF (radio frequency) or by microwave power of hundreds kHz (kilohertz) or several GHz (gigahertz). After etching, the remaining photoresist as well as residues formed during etch are removed, while desirably preserving the physical, chemical, and electrical properties of the ferroelectric material. Traditionally, post etch photoresist and residues are removed with relatively high temperature plasma ashing processes using fluorine or chlorine gas mixtures, wet chemical stripping processes, or a combination thereof. However, the ferroelectric characteristics are easily deteriorated with current high temperature plasma ashing processes as well as during prolonged contact with the wet stripping process chemicals. This deterioration results in decreasing the reading and writing performances of FeRAM devices and reducing the lifetime of the device.
It is important to note that plasma ashing processes significantly differ from etching processes. Although both processes may be plasma mediated, an etching process is markedly different in that the plasma chemistry is chosen to permanently transfer an image into the substrate by removing portions of the substrate surface through openings in a photoresist mask. The etching plasma generally includes high energy ion bombardment at low temperatures to remove portions of the substrate. Moreover, the portions of the substrate exposed to the ions are generally removed at a rate equal to or greater than the removal rate of the photoresist mask. In contrast, ashing processes generally refer to selectively removing the photoresist mask and any polymers or residues formed during etching. The ashing plasma chemistry is much less aggressive than etching chemistries and generally is chosen to remove the photoresist mask layer at a rate much greater than the removal rate of the underlying substrate. Moreover, most ashing processes heat the substrate to temperatures greater than 200° C. to increase the plasma reactivity. Thus, etching and ashing processes are directed to removal of significantly different materials and as such, require completely different plasma chemistries and processes. Successful ashing processes are not used to permanently transfer an image into the substrate. Rather, successful ashing processes are defined by the photoresist, polymer and residue removal rates without affecting or removing layers comprising the underlying substrate.
Current plasma ashing processes are especially unacceptable for use during fabrication of polymer FeRAM devices due primarily to the use of high temperatures (greater than about 200° C., as noted above), which deleteriously affect the electrical and chemical properties of the polymer ferroelectric material. Most prior art phototresist removal processes for ferroelectric device fabrication, if not all, describe a wet chemistry process (with no plasma ashing) to provide complete removal of the photoresist and residues. However, throughputs using wet chemistry alone are generally slow and inefficient resulting in prolonged contact times between the liquid chemicals employed in the wet chemistry processing and the substrate. As a result, chemical attack of the ferroelectric material can occur as well as the introduction of defects due to direct contact of the wet chemicals with the substrate.
Accordingly, it is desirable to have a photoresist and residue removal process for use with substrates comprising ferroelectric materials that do not deleteriously affect the properties of the ferroelectric material. The removal process should be non-oxidizing and non-damaging to the polymer ferroelectric materials, employ temperatures that do not affect the properties of the polymer ferroelectric material, and be amenable to high throughput processing as is highly desired for semiconductor manufacturing processes. Moreover, the removal process should minimize or eliminate any wet chemical processing since wet chemical processing has a propensity of introducing defects as a result of the direct contact of the chemicals with the substrate.